THE GOOD
1) Tremendous variety
Huge and interesting variety of possible wizard customization options, racial choices, heroes, and randomized maps with two planes of existence
2) Great sense of exploration
Randomized maps (again), the two planes, wide range of creatures, spells, events, artifacts. Filled to the brim with stuff to encounter, acquire, or battle
3) Broad scope
You go from being a puny and weak spellcaster with a small hamlet to a mighty wizard with a multi-plane empire
4) Interesting choices
What school of magic do you specialize in? What sorts of troops do you build? Do you use regular troops, heroes, artifacts, summoned creatures, combat magic or support magic to win fights? How do you place your cities? What spells do you use against your enemies empires, and what spells do you boost your own with? Where do you expand or conquer?
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There were some pretty significant negatives worth mentioning however
THE BAD
1) Terrible AI.
Somewhat forgiven for its age and the complexity of the game at the time, but I don't think I'd be so forgiving today. I don't even really care if the AI 'cheats' or doesn't even necessarily play by the same rules as the player, I'd rather have an interesting game, where the AI makes use of all aspects of the game, whatever methods are used behind the scenes to achieve that end.
2) Awful, tedious micromanagement.
City development was repetitive and tiresome in the mid and late game, and managing the huge numbers of troops also became a pain in the mid and late game.
3) Bugs.
Many, many bugs. Again, not surprising based on the scope of the game. Not too worried about that with Elemental though, pretty good track record so far!
4) Opaque diplomacy.
Again, I can't criticize this too much based on the age of the game, but AI diplomacy needs to be _very_ transparent, so that it's simply another aspect of gameplay, rather than an obscure, behind the scenes mechanism that is difficult to understand without access to a detailed wiki entry.
And I never want to see round robin tech tree trading abuse in another TBS again, ever 
5) Slow pace
The counterpoint to the micromanagement, the early game often had long stretches of 'next turn'.
I'd much, much rather see the length of the game compressed somewhat so the start is quicker, and the interesting bits come more quickly for midgame, culminating in a more epic end game.
Too many turn based strategy games bog down in the mid and late game, and I know there are a thousand thousand unfinished games of Moo/Mom/Civ/GalCiv out there.